Is there a suspicion of usury in the the fixed profit rate of the "Namaa" investment product at the Islamic Bank, and is it permissible to continue with it?

It is permissible to deal with Islamic banks and invest money with them, as long as their transactions are based on a Shariah supervisory board. The fact that the profit margin is almost fixed does not affect the validity of the transaction, because banks take precautions in their dealings and conduct studies to anticipate the profit margin, but they do not commit to paying a specific amount to the investor. It is preferable to consult scholars specialized in your country to inquire about the status of banks and the extent of their adherence to Shariah controls.